The present invention relates to a thermal airflow sensor, more particularly to a thermal airflow sensor for measuring flow rates of air intake in an internal combustion engine.
Conventionally, a thermal air-flow rate measuring device has been used widely as an airflow sensor in the air intake passage of an internal combustion engine for automobiles and the like because this type of air-flow rate measuring device can directly detect mass airflow rates. Particularly, up-to-date air-flow rate measuring devices fabricated by a semiconductor micro-machining technology have quick responses by which the air-flow rate measuring devices are also expected to be available to detect reverse flows of air in internal combustion engines.
For example, an embodiment which uses a thermal air-flow rate measuring sensor comprising a conventional semiconductor board has been disclosed in Japanese Non-examined Patent Publication No.9-503312 (1997). In this embodiment, the sensor element is supported by a metallic casing and the circuit board is housed in the casing. The semiconductor sensor element is electrically connected to the board by metallic wires.
However, this prior art has some problems to be solved. First, although the metallic casing containing a semiconductor sensor element and a control circuit board can increase the radio-noise resistance, the casing may have cracks at joints between the metallic casing and the silicone semiconductor sensor element when cold-hot cycles are applied repeatedly because their thermal expansion coefficients are greatly different. Second, as the metallic casing has a comparatively high heat conductivity, the engine heat will easily attack the semiconductor sensor element through the passage and the other components, which makes the characteristics of the sensor unstable. Further, the metallic casing has a complicated structure as illustrated in FIG. 5, Japanese Non-examined Patent Publication No.9-503312 (1997), which makes the product dispersed and expensive.
Furthermore, when the metallic casing is so constructed to be exposed to air in the passage of the engine room, the casing will be easily corroded under a severe environmental condition and the characteristics of the sensor may vary. Additionally, as the semiconductor sensor element in the casing is provided separately from the control circuit board, the number of parts to be assembled increases and consequently, the manufacturing cost becomes higher. Finally, as wires connecting the control circuit and the semiconductor sensor element must be provided in a module of this embodiment for assurance of reliability, the sensor may detect air disturbance caused by the passage wall and makes output noises greater, which makes the precision of the sensor unstable. (The semiconductor sensor element must be preferentially located away from the walls of the subsidiary walls.)
The main purpose of the present invention is to solve the above problems of the prior art and provide a high-reliability and low-production cost thermal airflow sensor almost free from characteristics errors.
A thermal airflow sensor for solving the above problems comprises
a semiconductor sensor element having a measuring part which comprises a thermoresistor formed on a semiconductor board and a heating resistor formed on the thinner part of said semiconductor board in a dent formed on a laminated ceramic board and
a control circuit which controls the intensity of a current to said heating resistor to make said heating resistor hotter by a preset temperature than said thermoresistor and gets an airflow signal indicating the flowrate of air in said laminated ceramic board. The dent can be provided by placing a punched board on a non-punched board.
Further, to increase the radio-noise resistance, a metallic plate separately from said control circuit is provided in said laminated ceramic board and grounded together with said control circuit. Said semiconductor sensor element and said control circuit are electrically connected by wire bonding and the wire-bonded part is covered with epoxy resin for protection. Said semiconductor sensor element and said resin-covered wire-bonded part are placed in the air passage to be in direct contact with airflows.
In accordance with the present invention, the thermal expansion coefficient of the laminated ceramic board is approximately equal to that of the semiconductor sensor element. This means that the sensor is very resistive to thermal stresses such as cold-hot cycles. Further, this ceramic sensor will not be corroded under such severe environmental conditions in the engine room. Further, as the control circuit and a metallic film for shielding electromagnetic waves are housed in the laminated ceramic board, the sensor product can have fewer parts. This improves the workability of the sensor products, makes the sensor products compacter and cheaper. Further, the dent in the laminated board can be provided just by placing a punched board on a non-punched board and requires no other processes such as bending. This means provision of high-precision low-cost dents.
The thermal airflow sensor in accordance with the present invention is free from disconnection and corrosion of wires which electrically connect the control circuit and the semiconductor sensor element because the wire-bonding part is wholly covered with epoxy resin for protection. Therefore, the wire-bonding part can be placed in the air passage and consequently, the semiconductor sensor element can be placed away from the walls of the subsidiary passage. This can suppress output noises and stabilize the output characteristics.
Further, as the semiconductor sensor element is not on the wall of the subsidiary passage, an engine heat through the wall of the passage will be hardly transferred to the semiconductor sensor element. Similarly, the heat conductivity of the laminated ceramic board is smaller than that of metal and the heat attack on the semiconductor sensor element is diminished. This can protect the sensor against change in characteristics by heat.